This invention relates to a system for controlling a variable-capacity compressor in an air conditioner by a so-call PI control.
In a conventional vehicle air conditioner provided with a variable-capacity compressor, the capacity or volume of the compressor is controlled by a computer in order that the actual temperature of the air at a downstream side of an evaporator can be brought close to a target temperature. More specifically, the capacity of the compressor is controlled by a solenoid valve incorporated in the compressor. The microcomputer effects a PI (proportional-plus-integral) control of electric current, supplied to the solenoid valve, in accordance with a deviation between the actual temperature and the target temperature, thereby controlling the capacity of the compressor.
In the above PI control, the integral control component contributes to the stability of the control, and the proportional control component contributes to a rapid responsibility. Therefore, by determining the above supply current in accordance with the sum of the two components, a good control of the compressor can be achieved. However, under a special condition in which the operation continues with the deviation between the actual temperature and the target temperature remaining unovercome for a long time, the integral control component causes adverse effects.
For example, if the air conditioner is operated in a hot day, in accordance with the large temperature deviation, the proportional control component is so determined as to increase the capacity of the compressor, and also the integral control component is gradually increased in such a direction as to increase the capacity of the compressor. Incidentally, the capacity of the compressor is limited by its mechanism or for other reasons. Even if the compressor is operated in its maximum capacity condition to bring the cooling ability of the air conditioner to its maximum level, it takes long time for the actual temperature to reach the target temperature. Therefore, when the actual temperature reaches the target temperature, the proportional control component becomes zero, but the integral control component tending to increase the capacity still remains as a large value. Therefore, the supply current has such a value that the capacity of the compressor much exceeds a level required to maintain the actual temperature at the target temperature. As a result, the actual temperature drops from the target temperature. In the process of dropping of the actual temperature from the target temperature, the proportional control component has such a value as to decrease the capacity; however, when the temperature deviation is small, the proportional control component is also small, and therefore can not cancel the integral control component. As a result, an excessive cooling is caused. And besides, it takes long time for the control to become stable after the excessive cooling.
Also, there is the following occasion. Namely, the actual temperature is higher than the target temperature, and the temperature deviation is not sufficiently reduced for a long time even when the capacity of the compressor is increased to the maximum level. In this condition, when the target temperature is changed to become higher than the actual temperature, the proportional control component is changed to the value tending to decrease the capacity of the compressor whereas the integral control component has already had the value much exceeding the value tending to increase the capacity of the compressor to the maximum level. As a result, the supply current, determined by the sum of the proportional control component and the integral control component, is maintained at the value tending to increase the capacity of the compressor to the maximum level, and is not soon changed in the direction to decrease the capacity of the compressor. As a result, the response is greatly delayed.
Japanese Laid-Open (Kokai) Patent Application No. 184517/88 discloses a control system in which the capacity of a compressor is subjected to a PID control. In this control system, when a vehicle is accelerated and also when a clutch for transmitting the rotation of an engine to the compressor is disengaged, the capacity of the compressor is decreased to the minimum level, and an integral control component is reset to a predetermined value (for example, zero). However, this prior art publication does not describe means for comparing the sum of the proportional control component and the integral control component with the limit value of the control amount corresponding to the maximum capacity and the minimum capacity of the compressor, and means for stopping the integral calculation when the above sum exceeds the limit value of the control amount and for determining the integral control component by subtracting the proportional control component from the limit value of the control amount.